Circulating system. — The function of cir culation, which is always so intimately con nected with that of respiration, presents, as might be supposed, two different conditions in the arachnidans. Those which breathe by means of trachea have not an apparent circu lation ; and in this respect they resemble in sects :—we attribute to them simply a dorsal vessel without any ramifications. Those, on the contrary, which possess branchial lungs, have an apparatus for circulation pretty well developed. It consists of an elongated vessel placed immediately beneath the integument along the middle line of the dorsal aspect of the back, on which account it has received the name of dorsal vessel (fig. 89). It is kept in its situation by small ligaments or muscles, (a a), which in insects are called aloe cordis. The texture of the dorsal vessel is membranous, and pretty firm ; it contains a colourless fluid. This heart is in communication with numerous vessels, but hitherto it has not been discovered which of these terminate in, or which arise from the organ, or, in other words, it is not known by what route the blood arrives at, or proceeds from the heart. We believe that we are able to dissipate the doubts which still exist as to this subject, but before we state our opinions we shall speak of the anatomical disposition of the apparatus. Treviranus has described it vaguely in the scorpions, but has well elucidated its structure in the spiders (aranece), more ticularly in Clubione atrox and Tegenaria mestica, .Fig. 89. In both these species merous vessels are observed to arise a from the heart, es- pecially from its posterior part (c c.) These proceed to a ramify indefinitely, d distributing them e selves over every organ; and we a have no doubt e with respect to d their true arterial nature. But in ad d dition to these ves sels there exist two a others of larger a size (d d,) which communicate in a one direction with a the heart, in an a other, by very fine ramifications, with the pulmonary branchiae. In Clu bione atrox these two vessels do not give out any branches in their course. No doubt remains in our mind but that these vessels maintain a direct communication between the heart and respiratory organs. The subjoined figure (fig. 89) will facilitate the understanding of these facts. It represents the heart and its appendages in the house-spider, ( Tegenaria domestica,) and shows the two canals which communicate with the heart and receive the srnall vessels (e e e e) that come from the pul monary branchim. Treviranus, to whom we owe these observations, has not, however, at tempted to explain the manner in which the circulation takes place in the arachnidans, and indeed this is to be determined by physiolo gical experiment and not by the dissection of the organs merely. The experiments which I have made, in conjunction with my friend 1\1. Milne Edwards, on the circulation of the crus taceans, enable me to give a satisfactory and doubtless true explanation of that of the arachni dans. The organs which exist in these animals, and we admit the precision of the anatomical facts detailed by Treviranus, are essentially the same as in the crustaceans. We find a heart, of the nature of which no one can entertain a doubt : then there are arteries proceeding from the heart and ramifying over every part of the body ; lastly, the heart receives on each side vessels which bring it into communication with the respiratory organs. These latter vessels are the analogues of the branchio-cardiac vessels of crustaceans. With respect to veins, of which the latter animals are destitute, they are equally wanting in the arachnidans, and are doubtless replaced by cavities of an irregular form which exist between all the organs of the body. Tre viranus, indeed, has remarked in the abdomen of Tegenaria domestica two small intervals which are discoverable through the integument, and in which he says the blood may be ob served to be collected. These reservoirs are perhaps the analogues of the venous sinuses of the Crustacea.
The nature of the vessels being thus deter mined, it becomes easy to conceive how the circulation takes place in the arachnidans the blood, leaving the heart, is distributed through all the arteries to the different organs for their nutrition : this being effected, and the nutrient fluid being thereby converted into ve nous blood, it begins to circulate through the sinuses before mentioned, and arrives by an insensible course at the pulmonary branchi.
There it is changed by contact with air into arterial blood, and returns to the heart by means of the branchio-cardiac vessels (e d) to be finally again propelled through the arteries (c.) Thus the ascertained anatomical facts, few as they are, permit us already to appreciate the mode of circulation in the arachnidans ; and we repeat that it is in every respect analogous to the circulation in the crustaceans.
Nervous system.—The nervous system is gangliated, as in all the articulate animals ; but it presents considerable differences of dis position in the different arachnidans : the scorpions in this respect vary much from the spiders.
In the Scorpionidce we find the following structure (fig. 90) :—the first ganglion, which is commonly called the brain (a), and which supplies the nerves to the parts of the mouth (b,c) is intimately blended with the nervous mass giving origin to the nerves of the legs (d). The succeeding ganglia are distinct from one an other, and are seven in number : the first three (1 2 3) are situated in the abdomen proper ; they have this peculiarity, that they are united together and with the ganglion, which may be termed cerebro-thoracic, by three instead of two chords of communication (e), which is the number found in all other articulate animals ; the four remaining ganglions (4 5 6 7) occupy the entire length of the post-abdomen, or that contracted portion of the body which is incor rectly termed the tail.
In the Aran mice the ganglions are fewer than in the Scorpionidee : d the first pair, or that which consti .
d tutes the brain, (fig.91, a,) is quite distinct from the thoracic ; these are e ftur in number (bb) but have under gone a remarkable degree of centrali zation, being inti mately connected together so as in deed to form a mass in which all traces of junction are lost, except at 3 the sides, which have remained free and in the form of smallconoid bodies directed outwardly so as to resemble, in the aggregate, the figure of a star. From the apex of each of these small • cones the nerve is given off to each leg. In the abdo men there does not exist any ganglion, but only a double longitudinal ner vous cord(c),which swells out at its ter mination. From this swelling (d) a great number of nerves (ee) pass off, which are distri buted to all the . . , organs contained in the abdominal cavity.
Organs of sense.
—We have no thing particular to observe with re spect to the smell or hearing of the arachnidans, for we are ignorant of the existence of these senses in the class, or at least of the parts of the body in which they are seated. With re gard to taste, the choice which the arachnidans make of their food sufficiently indicates that it exists in variable degree ; the organ is situated probably at the entrance of the pharynx. With regard to touch, the delicacy of that sense is in the ratio of the tenuity of the integument; but the extremities of the legs, and more especially of the maxillary palps seem to be expressly destined to bring the individual into relation with sur rounding objects. The sense of sight is the only one respecting which no doubt can exist ; particularly in the species which are the most perfect of the class, such as the spi ders, scorpions, &c. The eyes belong to that kind which are termed simple, in 'op position to those de nominated compound, and which are found exclusively in insects and crustaceans.
These simple eyes (ocelli) in arachnidans are two, four, six, or eight in number; they are situated on the an terior part of the body either superiorly or la terally. With respect to size they differ not only in different spe cies, but in the same individual, as in the and especially in the In the Scorpions (fig. 93) there are two eyes (a a) situated on the dorsal aspect of the cephalo-thorax, and closely approximated to the mesial line : these are of much larger size than the minute simple eyes (b b), which are placed on the sides and near the outer margins of the same segment. The two mesial eyes, on account of their size, have been selected by Miiller for the subject of his researches, which he published at Leipsic, and which have been translated by extract in the 17th volume of the first series of the " Annales des Sciences Natu relles." The following are the principal re sults of the labours of this accomplished na turalist.